Sputtering Processes
Magnetron sputtering is widely used for the deposition of thin films of various metallic and ceramic materials in production and laboratory applications. The cathode is mounted in a vacuum chamber, and is eroded on one of its surfaces by a DC or RF plasma discharge confined in close proximity to its surface by a closed loop magnetic tunnel. The eroded surface of the cathode is called the target, and is generally easily replaceable.
The target (typically a plate of the material to be deposited or the material from which a film is to be synthesized) is connected to the negative side of a DC power supply (or to an RF power supply). The positive side of the power supply may be connected to the vacuum chamber (if metal), or to a separate anode structure. The substrate is the object to be coated, and is placed or moved in front of the target and within several centimeters of it. The substrate may be electrically grounded, floating or biased, and may be heated, cooled, or some combination of these. An inert gas is introduced to provide a medium in which a glow discharge can be initiated and maintained. The most common sputtering gas is argon.
When the glow discharge is started, positive ions strike the target plate and remove mainly neutral target atoms by momentum transfer, and these condense on the substrate to form thin films. There are, in addition, other particles and radiation produced at the target and in the plasma (secondary electrons and ions, desorbed gases and photons) all of which may affect film properties.
It is desirable that the plasma density in the discharge be uniform over as much of the target surface as possible, in order to erode the largest possible fraction of the target volume. The plasma discharge is sustained by ionization of the sputtering gas, primarily by secondary electrons emitted from the target surface due to bombardment by positive gas ions and photons from the plasma. These electrons are trapped by the magnetic field, which in the prior art has the shape of a convexly arched tunnel, forming a closed loop on the front target surface and within the target volume, and which increases the ionization efficiency of the discharge current.
In some cases, gases or gas mixtures other than Ar are used. Usually this involves some sort of reactive sputtering process in which a compound is synthesized by sputtering a metal target (e.g., Ti) in the presence of a reactive gas (e.g., in an Ar-O.sub.2 mixture) to form a compound of the metal and the reactive gas species (e.g. TiO.sub.2).
The magnetic fields described in the prior art may be created by fixed or moving permanent magnets or electromagnetic coils. Generally, the field is created by placing magnets behind the back surface of the target, with their magnetization oriented perpendicular or parallel to the target surface, such that the resultant magnetic field above the magnets, and in the area of the target, is a convexly arched tunnel. Except for those portions of the magnetic field where the tangent to the flux lines is perfectly parallel or perpendicular to the target surface, at each position above the plane of the magnets the magnetic field has a parallel (to the target surface) and perpendicular (to the target surface) component.
The electric field due to the applied voltage (or the DC offset voltage in the case of an RF discharge) acting in combination with the parallel magnetic field component, causes the electrons to gain a net velocity along the magnetic tunnel in the direction given by the E.times.B drift (as it is commonly referred to in the field of plasma physics). By shaping the magnetic means in a circular or racetrack shape, the electrons will travel in a continuous loop above the surface of the target, colliding with and ionizing the gas atoms present near the target surface. The shape of the electron path defines the portion of the target that will be sputtered and therefore eroded. The required magnetic flux density is generally greater than about 20 gauss at the target surface in the center of the tunnel, for the parallel magnetic field component.
The interaction of this velocity along the tunnel with the tunnel magnetic field causes another force on the electrons, in the direction perpendicular to both the magnetic flux lines and the velocity (-V.times.B). The component of this force due to the parallel magnetic field component pushes the electrons toward the target, confining them in the direction perpendicular to the target surface.
The perpendicular magnetic field component, however, interacts with the velocity along the tunnel to produce lateral forces on the electrons, parallel to the target surface and perpendicular to the path of travel. In a conventional convexly arched tunnel, these lateral forces "pinch" the electrons toward the center of the tunnel from both sides. This pinching causes the plasma density and therefore the sputtering erosion to be highest at the center of the tunnel. As the sputtering erosion proceeds into the target volume, the magnetic field at the bottom of the erosion groove, and in particular its perpendicular component, becomes increasingly stronger, causing stronger pinching and resulting in an erosion groove in the target which is typically V-shaped. The fraction of the target volume which has been vaporized by the time the bottom of the erosion groove reaches the back of the target, called the target utilization, is rather low (typically around 30%) for a conventional magnetron sputtering cathode apparatus.
Numerous efforts have been made to improve target utilization. U.S. Pat. Nos. 4,162,954 and 4,180,450 of Morrison improve utilization by flattening the curvature of the convexly arched field lines forming the magnetic tunnel. Other prior art designs involve more mechanical and/or electrical complexity than a simple permanent magnet assembly and flat plate sputtering target. U.S. Pat. No. 3,956,093 of McLeod enlarges the area over which erosion occurs by shifting the location of the center of the erosion groove in an oscillatory fashion, by application of a variable magnetic bias field to the static, convexly arched magnetron field, using an electromagnet coil. U.S. Pat. No. 4,444,643 of Garrett similarly enlarges the center of the erosion groove by physically moving the entire magnet assembly. U.S. Pat. No. 4,198,283 of Class, et al. describes a method of improved utilization using multiple target segments, with a special cross sectional shape, and using a convexly arched magnetic tunnel within the target volume.
Despite the extensive attention that has been focused on efforts to improve target utilization, significant improvements have been obtained only at the expense of additional electrical or mechanical complexity. Although spent targets may be reworked into new targets or salvaged, any significant increase in target utilization translates into substantial cost savings. In addition, increased target utilization enables longer production runs and less downtime spent in replacing targets.
None of the prior art efforts to increase target utilization have recognized the importance or possibility of utilizing magnetic tunnel fields containing concave flux lines--regions in which the perpendicular magnetic field component is directed such that the lateral forces on the trapped electrons push them away from the center of the tunnel, rather than pinching them toward it--in the area of the target. Consequently, although target utilization improvements have created spent targets with broader V-shaped erosion grooves, they have previously been unable to create broad flat regions of target erosion by the use of a simple static magnetic tunnel field.